The solution was wrong.

The first ten results were spam. “Download now!” screamed one site, but the “download” button led to a survey about phone plans. Another offered a .exe file disguised as a PDF. His antivirus screamed louder than his frustration.

Frustrated, Marco slammed the laptop shut. Then, slowly, he opened the textbook again. He re-derived the Laplace transform by hand. He checked the Routh array twice. At 2 a.m., he found his mistake: a missing negative sign in the feedback loop.

“How do you know?” Marco asked.

“Because the manual—the real one, from the publisher—has an error on that very problem. Third edition never fixed it. I leave it there on purpose.”